Photovoltaic (PV) cells, which are essentially semiconductor junctions are used extensively to generate electric power from the light energy such as the solar energy. The photons present in the incident sunlight is absorbed by the semiconducting material of the PV cell 100 and the electrons are knocked loose from the atom, allowing the electrons to flow through the material to produce electricity. Presently, photovoltaic cell arrays are widely used in energy conversion arrangements such as solar panels, for example.
A panel may comprise an array of PV cells and the PV cells convert the incident solar rays into electric values such as voltage and current. The photovoltaic cells have provided a very useful means to convert solar energy into electric power. FIG. 1A depicts an electric equivalent of a photovoltaic cell 100. The photovoltaic cell 100 may be represented as a combination of a current source 110, a diode 120, and resistors R130 and R140. The photovoltaic cell 100 absorbs photons present in the incident solar rays 105-A to 105-K and converts the energy associated with the photons into voltage V120 and/or current I-185. The current I-185 is provided by the Equation (1) below.I-185=(I-115)−(I-125)−(I-155)  Equation (1)
wherein I-185 represents output current, I-115 represents the photocurrent generated by the current source 110, current I-125 represents the diode current that flows through the diode 120, I-155 represents the current flowing through the shunt resistor R130 connected across the current source 110. The voltage V121 across the resistor R130 is given by Equation (2) below.V121=V120+(I-185*R140)  Equation (2)
The current I-125, which flows through the diode 120 is provided by Equation (3) belowI-125=Irevsat{exp(qVj/nkT)/−1}  Equation (3)
wherein Irevsat is the reverse saturation current, n is the diode ideality factor, q is the elementary charge, k is Boltzmann's constant, T is the absolute temperature.
Also, the current I-155 flowing through the shunt resistor R130 is given by Equation (4) belowI-155=V121/R130=(V120+(I-185*R140))/R130  Equation (4)
Equation (5) below is arrived at by substituting Equations (3) and (4) in Equation (1), Equation (5) indicates that an inverse relationship exists between the output current (I-185) and the temperature (T).I-185=I-115−(Irevsat{exp(qVj/nkT)−1})−(V120+(I-185*R140))/R130  Equation (5)
Unfortunately, the solar rays incident on the photovoltaic cells increase the temperature of the photovoltaic cell and the capacity of the photovoltaic cell to generate electric power is affected by the increase in temperature. The increase in the temperature is caused by the difference in energy between the photons and the silicon band gap that is converted into heat. As shown in Equation (2), the increase in temperature decreases the current I-225 and the current I-285 delivered by the photovoltaic cell 100. The voltage and the current generated by the photovoltaic cell decreases, for example, by 20% as the temperature increases by about 30 degree Fahrenheit. Also, the present day heat pump liquid heaters merely use the power generated by the photovoltaic cell arrays. Therefore, there is a need to increase the efficiency of the photovoltaic cell based heater pump liquid heater systems.
FIG. 1B illustrates a graph of current I-185 vs. voltage V120. The graph depicts a decrease in the voltage V120 and the current I-285 with an increase in the temperature T of the PV cell 100. The plot 140 depicts current I-185 plotted along Y-axis and the voltage V120 plotted along X-axis. The plot 150, 160, and 170 represents the variation of current I-185 versus voltage V120 for temperatures of 20 degree centigrade, 40 degree centigrade, and 60 degree centigrade, respectively. As the temperature of the photovoltaic cell 100 increases the efficiency of the photovoltaic cell 100 drops almost to 20% and to compensate for the drop in voltage V120, the present day solar panels add 20% additional cells to compensate for the drop in voltage V120. The usage of 20% additional PV cells increases the size and cost of the solar panels. Therefore there is a need to improve the efficiency of the PV cells.